1. Field of the Invention
This invention relates to a sheet feeder that is provided in an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus such as a facsimile, printer, copy machine or the like, there is provided a sheet feeder that contains and feeds sheets for recording to an image recording section. Generally, the sheet feeder comprises a cassette, a holding member and a plurality of pickup rollers. The cassette contains a plurality of sheets stacked therein. The holding member holds the uppermost sheet of the sheets contained in the cassette from above. The pickup rollers are arranged at intervals on a rotation axis in a direction perpendicular to the direction of feeding the uppermost sheet. While the pickup rollers are kept in contact with the uppermost sheet during one rotation, the pickup rollers feed the sheet during one rotation. In other words, the holding member is a holding claw that holds a corner of the stacked sheets at the downstream side in the direction of transporting the uppermost sheet, and the pickup roller is a round cam. When a projection provided on the periphery of the pickup roller presses the uppermost sheet during the rotation of the pickup roller and locomotion is applied to the sheet toward the downstream side of the transporting with frictional force generated due to the pressing, the uppermost sheet separates from the holding member and is fed forwards. The fed sheet is carried to a transport roller that is provided at the downstream side of the transporting direction against the cassette, and further carried and fed to the image recording section by the rotation of the transport roller.
This kind of sheet feeder has a problem that will be described below.
Recently, it has been required that a cassette to be provided in a sheet feeder should have a large capacity in correspondence with acceleration of an image forming apparatus. The number of sheets that are stacked and contained in the cassette has been increased to four or five hundred, or more.
Meanwhile, if sheets are stacked to be contained in a cassette and left to stand, the sheets are deformed due to the influence of temperature or humidity in the cassette, with the result that a central portion of the sheets in a direction perpendicular to the transporting direction rises and becomes warped. In a case where the capacity of stacked sheets in a cassette is small (roughly 250 sheets or less), warps arising in the sheets are not so serious as to influence transportation precision. However, as described above, as the size of the cassette is enlarged and thus the number of sheets to be stacked and contained in the cassette is also increased (roughly 400 to 500 sheets or above, generally), warps of the sheet are increased and thus the influence on the transportation precision cannot be disregarded.
More specifically, for a case where 500 sheets of standard recording paper are stacked and left lying in the cassette, for example, an actual warp amount was 6 mm or more, under the conditions of high temperature and humidity such as the temperature is 30 degrees Celsius and humidity is about 85% and the sheets are left for three days. In general, when humidity is high, a warp is caused at the center portion of the sheet and the center portion rises. When he humidity is low, both ends of the sheet warp and rise. In this case, holding members hold both ends of the sheet, and the center portion of the sheet warp rise by about 3 mm.
FIG. 7 shows a warped sheet being transported by a conventional sheet feeder. In FIG. 7, P, 1 and 2 represent the warped sheet, pickup rollers and transport rollers, respectively. The sheet P having warps extending in the transporting direction at the center portion of the sheet P as viewed in the direction perpendicular to the transporting direction is fed in the direction of the arrow by means of pickup rollers 1 and transported by means of transport rollers 2. During this operation, warped portions of the sheet P are crushed. Therefore, unusual noise is generated from the sheet P. In addition, wrinkles and skews are caused in the sheet P when recording an image. As specifically shown in FIG. 7, when a transporting path from the pickup rollers 1 to the transport rollers 2 is curved at sharp angle, warps of the sheet P are crushed heavily. Therefore, each phenomenon as described above is caused markedly.
It is an object of the present invention to provide a sheet feeder that prevents various malfunctions from occurring, the malfunctions being caused due to warps to the sheet when the sheets stacked in the cassette are fed and transported.
To achieve the above object, there is provided a sheet feeder comprising:
a cassette for containing a plurality of sheets stacked therein;
a holding member for holding the stacked sheets in the cassette from above;
at least one pickup roller arranged on a first rotation axis extending in a direction perpendicular to a transporting direction, in which the uppermost sheet is fed from the cassette and transported, said at east one pickup roller feeding and transporting the uppermost sheet while the pickup roller is kept in contact with the uppermost sheet during one rotation of the pickup roller; and
a rotatable subsidiary holding member arranged on a second rotation axis extending in the direction perpendicular to the transporting direction, rotated in synchronization with a rotation of the pickup roller and in the same direction as the pickup roller rotates, and contacting the uppermost sheet from above to hold the warp of the sheets during one rotation of the rotatable subsidiary holding member before the pickup roller contacts the uppermost sheet.
Since the rotatable subsidiary holding member contacts and holds the uppermost sheet from above so as to suppress a warp caused in the stacked sheets before the pickup roller contacts and feeds the uppermost sheet, is possible to fed and transport the uppermost sheet in a state that the warp of the sheets has been suppressed.
According to a second aspect of the present invention, there is provided a sheet feeder wherein a distance between the second rotation axis of the rotatable subsidiary holding member and that surface of the rotatable subsidiary holding member which contacts the uppermost sheet is equal to, or greater than a distance between the first rotation axis of the pickup roller and that surface of the pickup roller which contacts the uppermost sheet.
Since the rotatable subsidiary holding member holds the sheets deeply, a warp caused to the sheets is suppressed.
According to a third aspect of the present invention, there is provided a sheet feeder wherein the rotatable subsidiary holding member is separated from the uppermost sheet during one rotation of the rotatable subsidiary holding member before the pickup roller is separated from the uppermost sheet.
The rotatable subsidiary holding remember can hold the sheets without hindering the pickup roller from feeding the sheets.
According to a fourth aspect of the present invention, there is provided a sheet feeder wherein the rotatable subsidiary holding member is made of a low-frictional material, and when the rotatable subsidiary holding member is in contact with the uppermost sheet, friction between the uppermost sheet and the rotatable subsidiary holding member is suppressed.
Since the friction between the rotatable subsidiary holding member and the sheets is suppressed, the rotatable subsidiary holding member can hold the sheets without hindering the pickup roller from feeding the sheets.
According to a fifth aspect of the present invention, there is provided a sheet feeder wherein two of the pickup rollers are arranged individually on both sides of the uppermost sheet as viewed in a direction of the first rotation axis, the first rotation axis and the second rotation axis are arranged identical with each other, and the rotatable subsidiary holding member is arranged between the two pickup rollers, so that the rotatable subsidiary holding member is arranged on a center portion of the uppermost sheet as viewed in a direction of the second rotation axis.
Normally, since a warp of the sheets occurs in the center portion of the sheets, the warp can be suppressed effectively with the rotatable subsidiary holding member arranged on the center portion of the uppermost sheet.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.